Optical spectral signatures of dusty starburst galaxies

We analyze the optical spectral properties of the complete sample of Very Luminous Infrared Galaxies presented by Wu et al., and we find a high fraction (similar to 50%) of spectra showing both a strong H delta line in absorption and relatively modest [O II] emission Ce(a) spectral. The e(a) signature has been proposed as an efficient method to identify dusty starburst galaxies, and we study the star formation activity and the nature of these galaxies, as well as the effects of dust on their observed properties. We examine their emission-line characteristics, in particular their [O II]/H alpha ratio, and we find this to be greatly affected by reddening. A search for AGN spectral signatures reveals that the e(a) galaxies are typically H II/LINER galaxies. We compare the star formation rates derived from the FIR luminosities with the estimates based on the Ha line and find that the values obtained from the optical emission lines are a factor of 10-70 (H alpha) and 20-140 ([O II]) lower than the FIR estimates (50-300 M. yr(-1)). We then study the morphological properties of the e(a) galaxies, looking for a near companion or signs of a merger/interaction. In order to explore the evolution of the e(a) population, we present an overview of the available observations of e(a) galaxies in different environments both at low and high redshift. Finally, we discuss the role of dust in determining the e(a) spectral properties and we propose a scenario of selective obscuration in which the extinction decreases with the stellar age.